int led = 13;
void setup() {                
  pinMode(led, OUTPUT);     
  Serial.begin(9600);
  Serial.print("ready");
}
void loop() 
{
     if (Serial.available())
     {
       char ch1=Serial.read();
       if(ch1=='t')
     { 
       digitalWrite(led,HIGH);
       }
       if(ch1=='f')
       { 
         digitalWrite(led,LOW);
       }
     }
}